[PATCH 10/75] Revert "drm/i915: Workaround incoherence between fences and LLC across multiple CPUs"

From: Kamal Mostafa
Date: Tue Jul 30 2013 - 11:19:44 EST -stable review patch. If anyone has any objections, please let me know.


From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

commit 46a0b638f35b45fc13d3dc0deb6a7e17988170b2 upstream.

This reverts commit 25ff119 and the follow on for Valleyview commit 2dc8aae.

commit 25ff1195f8a0b3724541ae7bbe331b4296de9c06
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Thu Apr 4 21:31:03 2013 +0100

drm/i915: Workaround incoherence between fences and LLC across multiple CPUs

commit 2dc8aae06d53458dd3624dc0accd4f81100ee631
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Wed May 22 17:08:06 2013 +0100

drm/i915: Workaround incoherence with fence updates on Valleyview

Jon Bloomfield came up with a plausible explanation and cheap fix
(drm/i915: Fix incoherence with fence updates on Sandybridge+) for the
race condition, so lets run with it.

This is a candidate for stable as the old workaround incurs a
significant cost (calling wbinvd on all CPUs before performing the
register write) for some workloads as noted by Carsten Emde.

Link: http://lists.freedesktop.org/archives/intel-gfx/2013-June/028819.html
References: https://www.osadl.org/?id=1543#c7602
References: https://bugs.freedesktop.org/show_bug.cgi?id=63825
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
Cc: Carsten Emde <C.Emde@xxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
[ kamal: revert the 3.8.y-stable version of 25ff119:
This reverts commit b578b3a82d830e2170d403b1fb29b649e26a48fb. ]
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
drivers/gpu/drm/i915/i915_gem.c | 28 +++++-----------------------
1 file changed, 5 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 979d892..22b5784 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2671,35 +2671,17 @@ static inline int fence_number(struct drm_i915_private *dev_priv,
return fence - dev_priv->fence_regs;

-static void i915_gem_write_fence__ipi(void *data)
- wbinvd();
static void i915_gem_object_update_fence(struct drm_i915_gem_object *obj,
struct drm_i915_fence_reg *fence,
bool enable)
- struct drm_device *dev = obj->base.dev;
- struct drm_i915_private *dev_priv = dev->dev_private;
- int fence_reg = fence_number(dev_priv, fence);
- /* In order to fully serialize access to the fenced region and
- * the update to the fence register we need to take extreme
- * measures on SNB+. In theory, the write to the fence register
- * flushes all memory transactions before, and coupled with the
- * mb() placed around the register write we serialise all memory
- * operations with respect to the changes in the tiler. Yet, on
- * SNB+ we need to take a step further and emit an explicit wbinvd()
- * on each processor in order to manually flush all memory
- * transactions before updating the fence register.
- */
- if (HAS_LLC(obj->base.dev))
- on_each_cpu(i915_gem_write_fence__ipi, NULL, 1);
- i915_gem_write_fence(dev, fence_reg, enable ? obj : NULL);
+ struct drm_i915_private *dev_priv = obj->base.dev->dev_private;
+ int reg = fence_number(dev_priv, fence);
+ i915_gem_write_fence(obj->base.dev, reg, enable ? obj : NULL);

if (enable) {
- obj->fence_reg = fence_reg;
+ obj->fence_reg = reg;
fence->obj = obj;
list_move_tail(&fence->lru_list, &dev_priv->mm.fence_list);
} else {

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